Advances in thin film technology have enabled the development of sophisticated integrated circuits. This advanced semiconductor technology has also been leveraged to create MEMS (Micro Electro Mechanical System) structures. MEMS structures are typically capable of motion or applying force. Many different varieties of MEMS devices have been created, including microsensors, microgears, micromotors, and other microengineered devices. MEMS devices are being developed for a wide variety of applications because they provide the advantages of low cost, high reliability and extremely small size.
Design freedom afforded to engineers of MEMS devices has led to the development of various techniques and structures for providing the force necessary to cause the desired motion within microstructures. For example, microcantilevers have been used to apply rotational mechanical force to rotate micromachined springs and gears. Electromagnetic fields have been used to drive micromotors. Piezoelectric forces have also been successfully been used to controllably move micromachined structures. Controlled thermal expansion of actuators or other MEMS components has been used to create forces for driving microdevices.
Various MEMS devices have been developed that implement electrostatic force to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,339, entitled “Method for Making Rolling Electrode for Electrostatic Device”, issued on May 12, 1981, in the name of inventor Kalt. These type of devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
Micromachined MEMS devices have also utilized electrostatic forces to move microstructures. Some MEMS electrostatic devices use relatively rigid cantilever members, as found in U.S. Pat. No. 5,578,976, entitled Micro Electromechanical RF Switch”, issued on Nov. 26, 1996 in the name of inventor Yao. These types of cantilevered actuators fail to disclose flexible electrostatic actuators with a radius of curvature oriented away from the substrate surface. Other MEMS devices disclose curved electrostatic actuators; however, some of these devices incorporate complex geometries using relatively difficult microfabrication techniques.
Recent developments have led to simplified MEMS devices that utilize electrostatic forces to move structures. These devices, which are based on flexible membranes that embody electrodes, provide for ease in fabrication and can be processed using conventional MEMS fabrication techniques. See for example, U.S. Pat. No. 6,057,520, entitled “Arc Resistant High Voltage Micromachined Electrostatic Switch”, issued on May 2, 2000, in the name of inventor Goodwin-Johansson. The Goodwin-Johansson '520 patent is herein incorporated by reference as if setforth fully herein. By modifying the biasing capabilities of the flexible film actuator disclosed in the Goodwin-Johansson '520 patent it is possible to fabricate actuators having a radius of curvature such that the actuator will fully curl prior to applying electrostatic voltage and fully uncurl upon the application of electrostatic voltage.
Current electromagnetic radiation imaging devices, typically infrared (IR) imaging devices, such as night vision devices, forward looking infrared devices (FLIRs) and the like, implement mechanical chopper wheels as the means by which radiation signals are pulsed for submission to the detectors/pixels. These chopping mechanisms are necessary for imaging device detectors to modulate or chop the incident electromagnetic radiation. The need for chopping of the signal is especially apparent in pyroelectric detectors since electrical charge is generated in the pyroelectric material by a change in temperature. The change in polarization of the pyroelectric material is defined in terms of the temperature change as:ΔPi=piΔTwhere ΔPi is a change in polarization, pi is the pyroelectric coefficient and ΔT is the temperature change that the pyroelectric film detects corresponding to changes in the incident radiation.
Signal chopping is also beneficial for other electromagnetic radiation detector systems, preferably infrared detector systems, such as thermal bolometers that produce a change in resistance with temperature. The resistance change in a thermal bolometer is a direct current effect, versus the pyroelectric detector, which is an alternating current effect, so a shuttering or chopping device is not necessarily required for a bolometer detector. However, for systems needing high sensitivity or low noise, signal chopping is needed to periodically modulate the signal to prevent thermal drift and signal noise such that high sensitivities can be achieved.
Signal chopping is beneficial for cooled infrared detectors, such as photodetectors that detect photons in an optical signal, generate charge carriers in response to the photon energy and create a signal in the form of current or voltage. By comparison, photovoltaic detector materials generally create electron-hole pairs in the p-n junction formed in the material in response to photon energy, which creates a voltage signal. Photoconductive detector materials generally produce a change in electrical conductance in the material in response to the photon energy, which creates a current signal. Quantum well infrared photodetector materials (QWIP) generally produce electronics within the quantum well semiconductor layers producing a current signal in response to photon energy. Similar to thermal bolometers, photodetectors provide for direct current effects, so a chopper device is not necessarily required for a photodetector. However, for systems needing high sensitivity and/or low noise, a signal chopping device may be implemented.
While a chopping device provides for the signal to be “chopped” or blocked at a continuous frequency, generally in the range of 10 to 1000 Hz, a shutter device provides for the signal to be blocked at intermittent intervals to create “on” and “off” states. There is also a need to provide reliable, low-power, miniaturized shutters to create intermittent on/off states for electromagnetic radiation detector systems, preferably IR detectors. In military systems, there are often environmental conditions that can be hazardous to the detector or user if the detector is not turned off at the appropriate time. Such conditions may occur when the detector is directed toward the sun, a high power laser source, or other conditions, which would produce a high brightness of light or heat. The IR detector needs to be quickly shuttered off in these conditions. Conventional mechanical shutters are large in size, require high power to operate, and suffer from slow speed of operation. These devices are not easily integrated in battery-operated, miniaturized detector systems with low operating power requirements and minimal size and weight.
The typical mechanical shutters or chopper wheels that are currently used in such imaging devices tend to be bulky in size (e.g., 1 to 4 inch diameter wheels made of patterned germanium or machined metal), consume significant electrical power and are typically constructed separate from the associated detectors and pixels. In addition, chopper wheels are potentially unreliable and inefficient in modulating the electromagnetic radiation signals. These chopper wheels are limited to “chopping” the signal at a continuous frequency and do not provide the capability for intermittent operation. Mechanical shutters are limited in the speed at which they can provide an intermittent on/off signal (minimum cycle time >1 millisecond). Additionally, since the shutter or chopper wheel will typically be responsible for chopping an entire focal plane array of detectors/pixels, if the shutter or chopper wheel fails, the entire FPA of detectors is rendered inoperable.
A need exists to develop a chopping or shutter device for electromagnetic radiation signal detection that is simple in design and fabrication, consumes less space and electrical power in the detector system, and is more reliable and efficient than current devices. By incorporating MEMS technology, and more specifically electrostatically activated flexible film actuators as chopping or shuttering elements it is possible to design and fabricate a unitary structure that allows for further reduction in detector/pixel size as advances in the field of IR imaging devices occur. The electrostatic activation of such a device would provide significant size reduction and consume much less power compared with the typical chopping wheel and associated drive motor. Power consumed by the electrostatically activated MEMS chopper is about 2 mW at 100 Hz compared with a chopper wheel motor which consumes several Watts of power.
Additionally, such a device would provide for individual shuttering elements (i.e., actuators) to be associated with an individual detector/pixel or, alternatively, a parsed portion of the overall FPA. This would allow the IR FPA to remain operational if only a single chopper or shutter element was to fail. In the same regard, it would be possible to close off individual detectors/pixels or small subsets of detectors/pixels could be closed while the remainder of detectors/pixels remain open. In this instance, the closed pixels could then be referenced as the background temperature to subtract out possible noise or temperature fluctuations occurring in the FPA. As such this would provide for a means of noise reduction and compensation for temperature fluctuations in the radiation detector. Current mechanical shutter or chopper wheel mechanisms are incapable of providing such noise reduction and/or temperature fluctuation compensation. In a similar fashion, if temperature spikes in the array result in “hot spots” (i.e. an area of constant brightness) this area could be closed independent of the remaining detector/pixels to reduce the signal resulting from the temperature spike to background levels. Localized detectivity could also be controlled by operating subsections of the FPA at a different chopping frequency. Lower chopping frequency could be used for areas of the image requiring higher sensitivity, and higher frequency could be used for faster image scan rates for less sensitive areas. Specific detectivity of IR detectors is known to be dependent on chopping frequency.
Moreover, such a device would allow for the duty cycle of the shutter/chopper to be altered to increase the detector output. A device that is capable of insitu duty cycle variance allows for an increase in the resistance, voltage or current signal produced by the detector by increasing the time per cycle that detector elements are exposed to the incident radiation. With conventional chopping wheel mechanisms the duty cycle can only be varied by replacing the wheel with one that exhibits the requisite duty cycle characteristics. An intermittent on/off shutter device can also be provided with much faster on/off cycle times (<100 microseconds).
As such a need exists to develop an improved electromagnetic radiation shutter/chopper device, specifically an electrostatically activated MEMS device that will leverage the simplified MEMS fabrication techniques with the advantages of individual actuator design. Such a design will additionally provide signal noise reduction, signal gain, faster speed, sensitivity and duty cycle modulation, compensation for temperature fluctuations and background referencing capabilities.